The periodic shedding of vortices produced in highly sheared gas flows has been recognized as a source of substantial acoustic energy for many years. Experimental studies have demonstrated that vortex shedding from gas flow restrictors disposed in large, segmented, solid propellant rocket motors couples with the combustion chamber acoustics to generate substantial acoustic pressures. The maximum acoustic energies are produced when the vortex shedding frequency matches one of the acoustic resonances of the combustor. It has been demonstrated that locating the restrictors near a velocity antinode generated the maximum acoustic pressures in a solid propellant rocket motor, with a highly sheared flow occuring at the grain transition boundary in boost/sustain type tactical solid propellant rocket motors. The resultant pressure oscillations caused by the vortex shedding can lead to excessive thrust oscillations and motor vibrations, having a detrimental effect on performance.
In addition to a segmented solid propellant rocket motor, highly sheared flow separations producing vortex shedding can be generated in a variety of ramjet engines which include sudden flow area expansions, such as those generally found at the dump plane of the combustor inlet sections of coaxial inlet ramjet engines. Periodic vortex shedding could therefore be a significant source of acoustic instability in these types of engines. The oscillatory acoustic pressures generated at the combustor inlet could couple with the acoustic resonances of the inlet shock structure to produce pressure oscillations that result in periodic combustor inlet unstarting, reducing the gas flow through the engine, and detrimentally affecting thrust generation.